The present invention relates to a process for producing a gallium-containing solution having a high concentration of a gallium component from aluminum smelting dust.
Gallium is noted as a raw material for semi-conductors such as gallium-arsenic(GaAs) or gallium-phosphorus(GaP) in recent year.
Gallium is widely distributed all over the earth, but does not exist as highly concentrated ore. Therefore, gallium is presently obtained by the following processes:
(i) Gallium is recovered from Bayer liquor which is a highly concentrated alkaline solution dissolving Al(OH).sub.3. An electrochemical reduction process with a mercury cathode or a solvent extraction process is used for recovering gallium from the Bayer liquor.
The solvent extraction process developed by Rhone Poulenc Co. using Kelex 100(trademark of alkylated-8-hydroxy-quinoline)is well-known (See pages 439-442, Vol. 2. 1979, Proceeding of the International Solvent Extraction Conference and French Patent No. 7629009).
(ii) Gallium is recovered from aluminum smelting dust by flotation(Japanese Patent Publication No. SHO59-111919, June 28, 1984) or alkaline roasting(See British Patent No. 1527981).
(iii) Gallium is recovered from the residue of the zinc leach solution in a zinc smelting process, in which the residue is leached with sulfuric acid. Gallium is recovered by an extracting agent such as versatic acid or isopropyl ether(See pages 65-76, Proceeding of Fourth Joint Meeting MMIJ-AIME 1980, Tokyo).
Though gallium is recovered from the residue of a zinc leach solution (iii) in Japan, the major source of gallium is bauxite which is the raw material for making aluminum. However, the efficiency of recovering gallium directly from bauxite is low,for the gallium content in bauxite is as low as 50-100 p.p.m.
The process for recovering gallium from Bayer liquor is mentioned in process (i) above. Electrochemical reduction with a mercury cathode creates an environmental pollution problem because of the mercury. The solvent extraction process also has problems such as deterioration of the expensive extracting agent, loss by the decomposition and the suspension and contamination of the Bayer liquor itself with organics, because the Bayer liquor is highly caustic.
The complicated processes and expensive treating costs are required in process(i) because of the very low gallium content. Accordingly, aluminum smelting dust having a comparatively high gallium content is preferable as the raw material. Gallium contained in Bayer liquor exhibits the same behavior as aluminum. Almost all the gallium goes into alumina. The gallium concentration in alumina depends on ghe gallium content in the bauxite, which is usually 30 p.p.m.-100 p.p.m. Gallium in the dust created during the electrolysis by using alumina is highly concentrated to 800 p.p.m.-3000 p.p.m. The general composition of dust evolved from an aluminum electrolysis process is 0.08-0.30 wt.% of gallium, 0.5-2.0 wt.% of iron, 10-20 wt.% of aluminum, 10-20 wt.% of sodium, 10-30 wt.% of flourine, 10-30 wt.% of carbon and less than 1 wt.% of silicon, nickel, titanium, calcium, copper, magnesium, cobalt and vanadium.
British Patent No. 1527981 relates to treatment of the aluminum smelting dust, which contains 0.2% gallium at the most.
The process consists of roasting the dust at a temperature of 500.degree.-800.degree. C., after adding alkaline flux in a large excess (5 times of the dust weight, for example), followed by leaching the roasted mixture of dust and flux with water, and then precipitating the dissolved gallium by adding base metal powder (aluminum or magnesium) into the leach liquor.
However, the above-mentioned process has a practical limitation, because iron, aluminum etc. are also extracted with gallium at the same time, and expensive alkaline flux has to be added in an amount greater than the amount of dust for roasting. The cost of the alkaline flux may exceed the price of the gallium to be recovered.